The present invention relates to a method of and arrangement for an interferometer of the type of Michelson. Interferometers of this kind are used for spectroscopy in a wide region of electromagnetic radiation; examples are, for instance the spectral analysis in chemistry or astronomy (in the infrared region).
In principle these interferometers of Michelson consist in accordance with FIG. 1 of two plane mirrors S1 and S2 perpendicular to each other, one of which (S1) is fixed, the other one (S2) can be moved parallel to itself and a beamsplitter ST under an angle of 45 degrees relative to each of the mirrors, which beamsplitter divides the wave trains of radiation coming from a source Q and impinging upon said beamsplitter into two halves of equal amplitude, one of which halves is transmitted to the (here) fixed mirror S1, the other one is reflected to the movable mirror S2. Reflected by mirrors S1 and S2, the halves of radiation recombine at the beamsplitter ST and reach the detector D.
If the optical path through both arms of the interferometer (with mirrors S1 and S2) is of the same length, then the halves superpose positively and result in a high detector signal;if by displacing the movable mirror S2,the optical paths are different by just .lambda..sub.S /2 of a certain wavelength .lambda..sub.S, then the two halves superpose negatively, they cancel each other and the detector receives no radiation of wavelength .lambda..sub.S. If the mirror is moved continuously over a path of many wavelengths, then for all wavelengths .lambda..sub.n of the impinging radiation, coaddition, cancelling and all states between these take place in turns continuously according to .lambda..sub.n. The herewith received detector signal (the interferogram) is the Fournier transform of the spectrum of the impinging radiation; digitizing the interferogram and applying the Fourier transform to it results in the spectrum.
The spectral resolving power of an interferometer is proportional to the path difference of its arms, hence the further the movable mirror is displaced, the higher the resolving power of the instrument is.
In all these well known interferometers of Michelson or in modified versions, the necessary path difference is generated by a back and forth movement of one or even both mirrors of the interferometer. For example the mirror is moved with the aid of a sliding guide (occasionally by using refractive components), or a pendulum, where for successive measurements the mirror has to be moved continuously back and forth. The necessity of moving the mirror back and forth limits the attainable speed of measurements and therefore the time resolution of the measurement. Furtheron in general the instrument cannot be used for measurements when the mirror is moving backwards, because in stopping and reversing the movement, the knowledge of the position of the mirror gets lost. Usually the mirror position is measured with the aid of a laser source and its measurement is necessary to peform the Fourier transform (and also to digitize the interferogram). Moreover, this means however, that it is impossible to measure continuously, only discrete sequences of a continuous event which can be acquired.
High requirements are set on the mechanism of the mirror movement, because during measurement (that is during mirror movement) both mirrors of the interferometer must stay exactly perpendicular to each other, which demands a high effort especially for large mirror displacements (high spectral resolving power) and/or in the case of short wavelengths under investigation.
Also known are interferometers with refractive elements, where the path difference is generated by moving back and forth a wedge or two wedges, resp. a prism or two prisms in one or both arms of the interferometer. FIG. 2 shows the principle design of such an interferometer, where the two fixed mirrors S1' and S2' can be either plane mirrors or retro reflectors. K1' and K2' are two identical wedges (prisms) made of a material, which has a different refractive index n.sub.k than air.ST' is the beamsplitter which can be made in form of a coating, applied to the backside of one of the wedges K1' or K2', or can be placed between the two surfaces of the wedges K1' and K2', that are facing each other. From the source Q' the radiation emerges and after interference it will be measured by the detector D'.
This interferometer has equal pathlengths in both arms, when the distances from mirror S1' respectively S2' to the beamsplitter ST' are the same and simultaneously the wedges K1' and K2' are not displaced with regard to each other, respectively they must be symetric with regard to the beamsplitter ST'. If one of the wedges, for example wedge K2' as indicated in dotted lines in FIG. 2, is displaced along the beamsplitter ST', for instance in the direction of its vertex, then the radiation has to traverse distances of different lengths in air and in the wedge material with regard to the both arms of the interferometer; this results in different optical pathlengths as long as the refractive index n.sub.k of the wedge material is different from the refractive index n.sub.L of air. Therefore, by moving back and forth one of the wedges K1' and K2' in the described manner, it is possible to generate different optical pathlengths in both arms of the interferometer without changing the geometrical pathlengths. These considerations are based on the following equation: EQU d.sub.O =n.multidot.d.sub.g ( 1)
where:
d.sub.o =optical pathlength PA1 n=refractive index of material (wedge) PA1 d.sub.g =geometrical pathlength. PA1 (a) performance of a back and forth movement PA1 (b) therefore a limited speed of measurement PA1 (c) really uninterrupted measurements are not possible.
Known are different types of interferometers using refractive components; in all types the pathdifference is generated by moving back and forth one or more optical elements. The movement(s) must be performed with high precision, therefore much effort is necessary in bearing and driving the movable components.
In accordance with the above explanations according to the state of the art of practical interferometers applied methods and arrangements are considered to have certain disadvantages besides the fact, that they demand a relatively great effort,these disadvantages are:
This is caused mainly by the fact, that the movable elements have to be accelerated and stopped all the time. As another disadvantage is the fact, that due to the necessary bearing of the movable elements, an operation of the interferometer is possible in general only in horizontal position, at least not in any orientation.